This invention relates generally to electrographic transfer apparatus, and more particularly to transfer roller apparatus for applying a uniform electrical transfer potential to a receiver member to effect transfer of a transferable image to such member.
In a typical electrographic process for making reproductions, an electrostatic charge pattern having an image-wise configuration corresponding to information to be reproduced, is formed on the surface of a grounded insulating member. The charge pattern is developed by applying developer material to such pattern to form a transferable image on the insulating member. The developer material includes for example, thermoplastic pigmented marking particles which are attracted to the charge pattern by electrostatic forces. The transferable image is transferred from the insulating member to a receiver member, and permanently fixed to such receiver member to form the reproduction. Transfer is accomplished by electrically charging the receiver member to a level sufficient to attract the developer material from the insulating member to the receiver member, while the receiver member is in contact with the area of the insulating member carrying the transferable image. Electrical charging of the receiver member is commonly effected by ion emission, for example from a corona charger, onto the surface of the receiver member, or by contacting the surface of the receiver member opposite the insulating member with an electrically biased transfer roller.
An electrically biased transfer roller is suitable for use in an electrographic process where multiple related images are transferred in superimposed relation on to a receiver member to form a composite reproduction, such as in making a multi-color reproduction. In such a process the receiver member is tacked to the transfer roller so that such member is successively returned into registered contact with the related transferable images on the insulating member. Examples of an electrically biased transfer roller are shown in U.S. Pat. No. 3,633,543, issued Jan. 11, 1972 in the name of Pitasi et al, and U.S. Pat. No. 3,832,055, issued Aug. 27, 1974 in the name of Hamaker. Such transfer rollers have hollow electrically conductive cores covered with electrically conductive, resilient, porous (foraminous) material. A partial vacuum effective within the cores tack the receiver members to the cover material, at least at the transfer nip formed with an image-carrying insulating member. However, the porosity of the cover material tends to create discontinuities in the electrical transfer field, which results in incomplete or non-uniform transfer.